University of Babylon/College of Information Technology/ Information Networks Dept. /
First Class / First Semester/ Subject: Network Architecture / Lecture: 2
A network is a combination of hardware and software that sends data from
one location to another. The hardware consists of the physical equipment that
carries signals from one point of the network to another. The software consists of
instruction sets that make possible the services that we expect from a network.
When we communicate, we are sharing information. This sharing can be
local or remote. Between individuals, local communication usually occurs face to
face, while remote communication takes place over distance. The term
telecommunication, which includes telephony, telegraphy, and television, means
communication at a distance (tele is Greek for "far").
Data communications are the exchange of data between two devices via some
form of transmission medium such as a wire cable. The effectiveness of a data
communications system depends on four fundamental characteristics:
1. Delivery
: the system must deliver data to the correct destination.
2. Accuracy
: the system must deliver the data accurately.
3. Timeliness
: the system must deliver data in a timely manner.
4. Jitter
: it refers to the variation in the packet arrival time.
A data communications system has five components:
1. Message: The message is the information (data) to be communicated.
2. Sender: The sender is the device that sends the data message
3. Receiver: The receiver is the device that receives the message.
4. Transmission medium: The transmission medium is the physical path by which
a message travels from sender to receiver. Some examples of transmission media
include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.
Lecturer: Dr. Ahmed M. Al-Salih
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University of Babylon/College of Information Technology/ Information Networks Dept. /
First Class / First Semester/ Subject: Network Architecture / Lecture: 2
5. Protocol: A protocol is a set of rules that govern data communications. It
represents an agreement between the communicating devices.
Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex:
Simplex
In simplex mode, the communication is unidirectional, as on a one-way street.
Only one of the two devices on a link can transmit; the other can only receive.
Keyboards and traditional monitors are examples of simplex devices. The keyboard
can only introduce input; the monitor can only accept output. The simplex mode can
use the entire capacity of the channel to send data in one direction.
Half-Duplex
In half-duplex mode, each station can both transmit and receive, but not at the
same time. When one device is sending, the other can only receive, and vice versa.
The half-duplex mode is like a one-lane road with traffic allowed in both directions.
When cars are traveling in one direction, cars going the other way must wait. In a
half-duplex transmission, the entire capacity of a channel is taken over by whichever
of the two devices is transmitting at the time.
Lecturer: Dr. Ahmed M. Al-Salih
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University of Babylon/College of Information Technology/ Information Networks Dept. /
First Class / First Semester/ Subject: Network Architecture / Lecture: 2
The half-duplex mode is used in cases where there is no need for communication in
both directions at the same time; the entire capacity of the channel can be utilized
for each direction.
Full-Duplex
In full-duplex mode (also called duplex), both stations can transmit and
receive simultaneously.
The full-duplex mode is like a two-way street with traffic flowing in both directions
at the same time. In full-duplex mode, signals going in one direction share the
capacity of the link: with signals going in the other direction. This sharing can occur
in two ways: Either the link must contain two physically separate transmission paths,
one for sending and the other for receiving; or the capacity of the channel is divided
between signals traveling in both directions.
One common example of full-duplex communication is the telephone
network. When two people are communicating by a telephone line, both can talk and
listen at the same time. The full-duplex mode is used when communication in both
directions is required all the time. The capacity of the channel, however, must be
divided between the two directions.
Some terminology related to the operation of network devices is:
- Domain is a specific part of a network.
- Bandwidth is the amount of data that can be carried across a network in a given
time period.
- Unicast data is data meant for a specific device.
- Broadcast data is data meant for all devices; a special broadcast address indicates
this.
Lecturer: Dr. Ahmed M. Al-Salih
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University of Babylon/College of Information Technology/ Information Networks Dept. /
First Class / First Semester/ Subject: Network Architecture / Lecture: 2
- Multicast data is data destined for a specific group of devices; a special address
indicates this.
- A bandwidth domain, known as a collision domain for Ethernet LANs, includes
all devices that share the same bandwidth.
- A broadcast domain includes all devices that receive each other’s’ broadcasts (and
multicasts).
Network Fundamentals - Network Devices - Hub
•A typical Ethernet LAN uses unshielded twisted-pair (UTP) cables with RJ-45
connectors.
•Because these cables have only two ends, an intermediary device is needed to
connect more than two computers. That device is a hub.
•A hub works at Layer 1 and connects multiple devices so that they are logically all
on one LAN.
•NOTE : The physical connection point on a network device—a hub, switch, or
router—is called an interface or a port.
•A hub has no intelligence—it sends all data received on any port to all the other
ports. So, devices connected through a hub receive everything that the other
devices send, whether or not it was meant for them.
•All devices connected to a hub are in one collision domain and one broadcast
domain.
Lecturer: Dr. Ahmed M. Al-Salih
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University of Babylon/College of Information Technology/ Information Networks Dept. /
First Class / First Semester/ Subject: Network Architecture / Lecture: 2
•Note: A hub just repeats all the data received on any port to all the other ports; thus,
hubs are also known as repeaters.
Network Fundamentals - Network Devices - Switch
•LAN switches are Layer 2 devices and have some intelligence—they send data to
a port only if the data needs to go there.
•A device connected to a switch port does not receive any of the information
addressed to devices on other ports. Therefore, the main advantage of using a
switch instead of a hub is that the traffic received by a device is reduced because
only frames addressed to a specific device are forwarded to the port on which
that device is connected.
•Switches read the source and destination MAC addresses in the frames and
therefore can keep track of who is where, and who is talking to whom, and send
data only where it needs to go.
•If the switch receives a frame whose destination address indicates that it is a
broadcast (information meant for everyone) or multicast (information meant for
a group), by default it sends the frame out all ports (except for the one on which
it was received).
•All devices connected to one switch port are in the same collision domain, but
devices connected to different ports are in different collision domains. By
default, all devices connected to a switch are in the same broadcast domain
Network Fundamentals - Network Devices - Router
•A router goes one step further than a switch. It is a Layer 3 device that has much
more intelligence than a hub or switch.
Lecturer: Dr. Ahmed M. Al-Salih
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University of Babylon/College of Information Technology/ Information Networks Dept. /
First Class / First Semester/ Subject: Network Architecture / Lecture: 2
•By using logical Layer 3 addresses, routers allow devices on different LANs to
communicate with each other and with distant devices—for example, those
connected through the Internet or through a WAN.
•The logical Layer 3 addresses is the TCP/IP’s IP addresses..
•The router reads the source and destination logical addresses in the packets and
therefore keeps track of who is where, and who is talking to whom, and sends
data only where it needs to go.
•All devices connected to one router port are in the same collision domain, but
devices connected to different ports are in different collision domains.
•Routers block broadcasts (destined for all networks) and multicasts by default;
routers forward only unicast packets (destined for a specific device) and packets
of a special type called directed broadcasts.
•Routers work at the OSI model network layer. The main functions of a router are
first to determine the best path that each packet should take to get to its
destination and second to send the packet on its way.
•Sending the packet out the appropriate interface, along the best path, is also called
switching the packet because the packet is encapsulated in a new frame, with
the appropriate framing information.
Lecturer: Dr. Ahmed M. Al-Salih
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